This research focuses on an experimental investigation to identify the effects of fly ash on the electrochemical process of concrete during the curing time. A rebar was analysed using potentiostat to measure the rest potential, polarization diagram, and corrosion rate. Water-to-cement ratio and amount of fly ash were varied. After being cured for 24 hours at a temperature of 65°C, the samples were immersed in 3.5% of NaCl solution for 365 days for electrochemical measurement. Measurements of the half-cell potential and corrosion current density indicated that the fly ash has significant effects on corrosion behaviour of concrete. Although fly ash tends to create passivity on anodic current, it increases corrosion rate. The corrosion potential of this concrete mixture decreases compared to concrete without fly ash. From the result, it can be summarized that concrete mixture with 70% of OPC (Ordinary Portland Cement) and 30% fly ash has shown the best corrosion resistance.
In aggressive environments, early degradation of reinforced concrete structures is caused by steel corrosion. In concrete, steel is passive due to alkalinity of concrete which is protective of steel surface. However, effects of carbon, chlorine, and acid conditions can damage the passive film which makes reinforcing steel exposed to the active environments to corrode. Some efforts have been conducted to prevent the corrosion of reinforcing steel by improving the quality of the concrete. Recently, the uses of polymer to improve quality of concrete have attracted and obtained great attention. Combination of concrete with polymer, so-called geopolymer, has advantages such as good tensile strength, light weight, high corrosion resistance, and durability.
Therefore, in recent years, geopolymer concrete has become a potential alternative to replace the conventional Portland cement concrete (OPC) used in the infrastructure construction. In contrast, with OPC, most geopolymer systems rely on the minimally processed natural materials to provide the binding agents. Geopolymer is based on the chemistry of alkali activated inorganic binders. This chemistry is involved in antique binders [
Some researchers such as Perná and Hanzlíček [
Most of the studies on geopolymer have shown relationship of fundamental aspects of chemical and binder system on concrete strength, yet the role of concrete in preventing reinforcing steel corrosion has not yet been fully understood. Hence, to improve the corrosion resistance of concrete, environment-friendly concrete which is geopolymer will be introduced. In this study, the corrosion rate tests and corrosion polarization tests of reinforcing steel concrete were tested in 3.5% NaCl concentrations and were investigated. This study was also conducted to describe ability of geopolymer concrete combined with fly ash to protect reinforcing steel bar on corrosion.
The ratios of fly ash to the cement used in the mix design were 0%, 10%, 30%, and 50% noted as A, B, C, and D in Table
Mixture proportion of geopolymer concrete.
Water | Cement | Fly ash | Fine aggregate | Coarse aggregate | |||
---|---|---|---|---|---|---|---|
OPC | A | 1.09 | 1.84 | 0 | 3.88 | 5.62 | |
OPC + 10% FA | B | 1.09 | 1.65 | 0.19 | 3.88 | 5.62 | |
OPC + 30% FA | C | 1.09 | 1.29 | 0.55 | 3.88 | 5.62 | |
OPC + 50% FA | D | 1.09 | 0.92 | 0.92 | 3.88 | 5.62 | |
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NaOH solution | Na2SiO3 solution | Fly ash | Fine aggregate | Coarse aggregate | Extra water | ||
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Geopolymer concrete | E | 0.25 | 0.54 | 2.15 | 3.34 | 6.22 | 0.16 |
Concrete properties and mix design.
Strength | 25 N/mm2 |
Aggregate type: coarse | Crashed |
Aggregate type: fine | Crashed |
Free water-cement ratio | 0.59 |
Slump: 30–60 mm, VB (time) | 3–6 second |
Max aggregate size | 20 mm |
Free water content | 210 kg/m3 |
Cement content (C1) | 355 kg/m3 |
Concrete density | 2400 kg/m3 |
Total aggregate content | 1834.09 kg/m3 |
Fine aggregate content | 3514.7 kg/m3 |
Coarse aggregate content | 1052.10 kg/m3 |
Curing time | 28 days |
The concrete specimens were placed in a shaded area in room temperature. These specimens were protected from the exposure to sunlight and rainfall. The specimens were subjected to immersion in the artificial seawater that contains sodium chloride (NaCl) with 3.5% concentration. The specimens were immersed in the solution for 365 days.
The corrosion rate of the reinforcement steel bar was determined by the linear polarization resistance (LPR). The electrochemical tests were performed on the Ordinary Portland Cement concrete, pozzolan concrete, and fly ash-based geopolymer concrete specimens using potentiostat. LPR measurements are generally used to determine the instantaneous corrosion rate of an electrode. The IR drop value in the cover concrete is significant and may vary among the specimens as concrete is a high resistive medium. The IR drop values of the concrete have to be determined and compensated for determining the corrosion current density in mAm−2 relative to steel area. The linear polarization resistance is defined as the slope of this curve (
Three electrodes connected with the potentiostat were working electrode, reference electrode, and counter electrode. The rebar was embedded in the concrete as the working electrode. The saturated calomel electrode used was an electrode made of silver immersed in saturated potassium chloride (KCl) solution. A carbon rod was the counter electrode. Figure
Experiment setup of the corrosion test.
Setup of corrosion test.
The results of the calculation are tabulated in Table
Effects of types of concrete on corrosion rate and corrosion potential.
Types of concrete | Corrosion potential ( |
Corrosion current ( |
Corrosion rate ( |
---|---|---|---|
A | −0.539 | 1.2512 | 0.0154 |
B | −0.549 | 1.1258 | 0.0131 |
C | −0.574 | 1.2208 | 0.0142 |
D | −0.585 | 1.0908 | 0.0126 |
E | −0.670 | 1.0728 | 0.0264 |
The pozzolan concrete (samples B, C, and D) contains a different percentage of cement and fly ash that has low corrosion rate compared with the Ordinary Portland Cement concrete and geopolymer concrete. With the presence of fly ash, it helps to control the alkali-silica reaction by reducing the permeability to water and the diffusivity to alkali supplied by external sources from the seawater or sodium chloride solution. The pozzolanic reaction, in which calcium hydroxide formed on the hydration of the cement reacts with silica in the supplementary cementing material to form calcium silicate hydrate, fills in the pores and reduces their connectivity.
From Figure
Effects of rebar on corrosion potential in 3.5% NaCl solution after immersion for 365 days.
Figure
Polarization scan of rebar in 3.5% NaCl solution after immersion for 365 days.
All of the concrete samples A, B, C, D, and E showed the presence of the stable passive film formed in the circle. The formation of a passivating oxide film on metal surfaces is an important aspect of corrosion protection [
Based on Figure
Corrosion rate of several types of concrete in 3.5% NaCl solution after immersion for 365 days.
From the results, it can be summarized that concrete mixture with 70% of OPC (Ordinary Portland Cement) and 30% fly ash had the best corrosion resistance reinforcing steel bar. It gave the lowest corrosion rate. Scanning polarization showed that geopolymer increased corrosion potential to the magnitude of 50 mV. When fly ash was combined with geopolymer, the concrete indicated decrease of corrosion potential. The formations of passive films on steel surfaces were also found in geopolymer concrete. When the more fly ash concentration is contained in the geopolymer concrete, the tendency for formation of passive films on the rebar is higher. Geopolymers concrete has given positive impacts on anodic polarization on the steel. However, due to low resistivity of fly ash, it caused increase of corrosion rate on the steel. The lowest corrosion rate achieved by this mixture was 6.248 × 10−3 mm/year on the 60th day of immersion test. Meanwhile, the geopolymer concrete had a corrosion rate of 71.312 × 10−3 mm/year. The corrosion potential that was shown by geopolymer concrete was −0.905 mV.
The authors declare that they have no competing interests.
The authors are thankful to Universiti Malaysia Pahang for providing grant and facilities for the research.